The present invention relates to a method of electric sintering and a mold for use in such method, and relates more particularly to the art of electric sintering utilizing plasma discharge, pulsating current, etc.
More specifically, the invention relates to an electric sintering mold having a clamping portion capable of clamping the powder material, the clamped material being sintered by the joule heat generated within the material in response to an externally supplied pulsating current and a pressure applied to the material from a pressurizer. The invention also relates to an electric sintering mold of a type including a die defining a cavity for receiving the powder material and a punch capable of advancing into the die cavity. The invention relates also to an electric sintering method using such mold. The invention further relates to an electric sintering apparatus including a die defining a cavity for receiving the powder material, a punch capable of advancing into the die cavity, a pair of electrodes capable of sending a current to the powder material received within the die, and a power supply unit capable of supplying a pulsating current to the pair of electrodes.
In the art of electric sintering described above, for reducing the time required for sintering the powder material, the prior art has proposed a method of sintering the powder material by utilizing joule heat generated within the material in response to a pulsating current applied to the material in cooperation with a pressure also applied to the material from a pressurizer. Referring more particularly to this method, the powder material is charged in a die and then this die holding the material therein is clamped between a pair of upper and lower punches, and the material is pressurized and at the same time the pulsating current is applied to the layer of the powder material within the die, whereby joule heat is generated within the material, which heat, in cooperation with the pressure, sinters the material. With such electric sintering method, the time required for sintering the material may be reduced advantageously, in comparison with the more conventional method of sintering material in furnace atmosphere which requires hours until completion of sintering.
The sintering mold employed for such method as above requires high electroconductivity for allowing the externally supplied current to be smoothly conducted to the material via the mold and requires also sufficient mechanical strength under high temperature condition since the mold must be able to withstand the high temperature generated in the material held within the mold and must also be able to transmit the high pressure from the pressurizer to the material held within the mold.
Then, as material suitable for forming such mold satisfying both of the requirements of high electroconductivity and high mechanical strength under high temperature condition, the prior art has proposed e.g. graphite or WCxe2x80x94Co which is a superhard material.
In recent years, there is an increasing demand for forming products or components by means of sintering. In particular, such components as a piston head for an automobile engine has been manufactured by sintering. In this regard, with the conventionally proposed electric sintering method described above, if the material to be sintered is highly conductive material such as aluminum, a significant electric current density is required to obtain a large amount of joule heat. Hence, unless the electric power supply unit is capable of supplying an extremely large amount of current, it will take a long time for the material to reach its sintering temperature. With a typical power supply unit, the sintering operation takes as much as half an hour to be completed. In this manner, according to the conventional art, if improvement in the turn around time is desired, this is possible only with enlargement of the system and resultant increase of system costs. That is, in quest for more efficient sintering suitable for mass-produced articles, there has been the continuing need for minimizing their processing cycle. And, this should be made possible without inviting enlargement of the system, from the view point of manufacture costs.
In addition, the conventional electric sintering mold made of graphite, WCxe2x80x94Co or the like has the further disadvantage that the inner surface of the mold tends to erode gradually due to physical and/or chemical reaction occurring in the powder material when placed under the high temperature and pressure condition therein.
For this reason, in order for the mold to be usable for a plurality of times while maintaining its inner dimension, that is, as high as possible dimensional accuracy of the compact to be obtained therefrom, it would be needed to apply a mold releasing agent such as boron nitride (BN) powder or spray or carbon powder to the inner surface of the mold (generally, to the inner surface of the die and also to the pressing surface of the punch) for each run prior to charging of the material therein. More particularly, after completion of each sintering operation, before starting the next run, the operator must additionally carry out the troublesome maintenance operation of checking the inner dimension and the surface condition of the mold and then reapplying new releasing agent when he/she finds the mold unusable for the next run. In this respect, there remains room for improvement.
Moreover, even with use of such releasing agent, the conventional graphite or WCxe2x80x94Co mold still has a rather limited service life, which is unsatisfactory from the economical point of view. Presumably, this is because the releasing agent cannot fully block the physical and/or chemical reaction of the charged powder material occurring under the high temperature and pressure condition.
Accordingly, in view of the above-described shortcomings of the prior art, a primary object of the present invention is to provide a further improved electric sintering method and apparatus which enable highly efficient electric sintering operation by minimizing the time required for sintering operation without increasing the current capacity of the power supply unit, providing good releasing of the molded product from the mold after sintering without the need of applying a releasing agent prior to charging of the power material for sintering therein, and also by providing longer service life than the conventional graphite or WCxe2x80x94Co type electric sintering mold.
For accomplishing the above-noted object, according to one aspect of the invention, there is provided an electric sintering mold which contains metal boride having electroconductivity.
For example, this electric sintering mold of the invention may be provided in the form of a compact containing metal boride having high electroconductivity, plus other optional component such as refractory material (e.g. oxide such as SiO2, Al2O3, etc; carbide such as SiC; nitride such as SIALON, Si3N4, etc.). Then, with this mold, the electric current externally supplied thereto may be converted in a very efficient manner through this mold into joule heat to be generated within the powder material held therein. Further, as this mold has a higher mold-releasing performance than the conventional graphite or WCxe2x80x94Co molds, the invention""s mold is free from the need of applying a releasing agent to the mold prior to charging of the power material therein. Moreover, even without application of such releasing agent at all, this mold can still provide greater durability, i.e. longer service life than the conventional molds described above.
According to another aspect of the invention, there is provided an electric sintering mold comprising: a die defining a cavity capable of receiving powder material therein; and a punch capable of advancing into the cavity of the die, the powder material held within the cavity of the die being subjected to a pressure from the punch and also to an externally supplied pulsating electric current so that joule heat is generated within the powder material for sintering the material; wherein at least one of the punch and the die is made of a material which contains metal boride having electroconductivity.
In this case too, the electric sintering mold of the invention may be provided in the form of a compact containing metal boride having high electroconductivity and other additional component such as refractory material (e.g. oxide such as SiO2, Al2O3, etc; carbide such as SiC; nitride such as SIALON, Si3N4, etc.). Then, with this mold, the electric current externally supplied thereto may be converted in a very efficient manner through this mold into joule heat to be generated within the powder material held therein. Further, as this mold has a higher mold-releasing performance than the conventional graphite or WCxe2x80x94Co molds, the invention""s mold is free from the need of applying a releasing agent to the mold prior to charging of the power material therein. Moreover, even without application of such releasing agent at all, this mold can still provide greater durability, i.e. longer service life than the conventional molds described above.
According to the invention, the material forming the die and/or the punch has an electric resistivity ranging from 10xc3x9710xe2x88x927 to 10xc3x9710xe2x88x921 (xcexa9cm). This setting provides even more efficient conversion of the pulsating current into the joule heat within the powder material held in the mold.
Also preferably, according to the invention, the material forming the die and/or the punch has Vickers hardness ranging from 10 to 50 (GPa). This setting provides the material with even higher mechanical strength for restricting xe2x80x9cbiting-inxe2x80x9d of the powder material into the inner surface of the mold in response to the pressure applied from the pressurizer, thus achieving still longer useful life of the mold as well as higher dimensional accuracy in the sintered compact obtained.
Preferably, according to the invention, the metal boride comprises titanium diboride. Titanium diboride is most suitable for its low electric resistivity and high Vickers hardness.
According to a still further aspect of the present invention, there is provided an electric sintering method characterized in that the powder material is preheated prior to the sintering operation thereof within the die.
With the above method, the powder material is preheated prior to its sintering operation. Hence, this method can reduce the time required for heating the material up to the sintering temperature, so that the sintering operation may be completed within a very short time period.
Preferably, according to the invention, in the method described above, the powder material is preheated to a temperature which is below the fusing temperature of the powder material and which also is higher than 40% of the electric sintering temperature in the Celsius scale.
If the powder material is preheated in the range specified above, the time period required for sintering operation may be further reduced. In addition to this advantage of speeding up the sintering operation, the preheating of the powder material provides another advantage of reducing the deformation resistance of the material so as to make it easier for the material to be compressed with higher density. Incidentally, if the preheating temperature is set lower, this will prevent disadvantageous growth of large metal crystals during this preheating operation. However, if the preheating operation is completed within a short time period, this will not allow time for growth of such large metal crystals. Therefore, disadvantageous enlargement of metal crystals may be avoided even with high preheating temperature. For this reason, its is preferred that the preheating operation be completed within the shortest possible time period at the highest possible temperature. In this respect, it should noted, however, that the preheating temperature should not be as high as or even too near the fusing temperature of the powder material so as to avoid xe2x80x9cprematurexe2x80x9d sintering of the material at this preheating stage. If the current and pressure are applied the powder material after such preheating operation thereof, this powder material may be sintered within a very short time period.
According to the invention, the preheating operation is effected on the die holding the powder material therein prior to its electric sintering operation.
The above construction can prevent cooling of the preheated powder material by the die. Hence, the subsequent operation of externally supplying the electric current to the preheated powder material may be effected even more efficiently. Consequently, the sintering time period may be still further reduced.
According to a still further aspect of the invention, there is provided an electric sintering apparatus wherein the die thereof includes preheating means, as second heating means, capable of preheating the powder material held in the die or the die per se and then maintaining the powder material at the preheated temperature until the subsequent electric sintering operation of the powder material.
With the above-described construction, when the powder material held in the die may be preheated and then maintained at the preheating temperature without being cooled until its sintering operation. Hence, the subsequent electric sintering operation may be carried out efficiently in a further reduced time period. As described hereinbefore, in addition to the reduction in the sintering time, the preheating provides the further advantage of reducing the deformation resistance of the powder material, which allows higher density of the material when compacted. Further, if the sintering operation is effected under vacuum, the powder material may be charged into the die disposed inside the vacuum chamber. Then, this material may be pressurized by the punch and supplied with the current to be sintered thereby. In such case, since the preheating means is incorporated within the die, when the powder material is preheated within this die, the heat-resistive layer may be formed thin, so that good heating efficiency may be maintained.
Consequently, the invention has achieved its primary object of providing an electric sintering method and apparatus suitable for mass-production, by reducing the cycle time of the sintering process without inviting increase in the current capacity of the electric power supply unit.
According to the invention, in the electric sintering apparatus described above, the apparatus may include the electric sintering mold.
With the above construction, namely, if the electric sintering mold, is provided in the invention""s apparatus capable of reducing the cycle time of sintering operation by preheating the powder material held in the die or the die itself without increasing the current capacity of the power supply unit, the same functions and effects as the mold described hereinbefore may be attained, so that such apparatus may effect its electric sintering operation even more efficiently.
Further and other aspects, features and advantages of the invention will become apparent from the following detailed description of the preferred embodiments thereon in conjunction with the accompanying drawings.